The present invention relates to a directed reflector, such as a retroreflector and more particularly to, directed reflectors which utilize an array of lenslets in combination with a reflector such that incident (incoming) light of a wide range of acceptance angles is reflected therefrom at a generally constant return angle relative to the angle of incidence.
Directed reflectors are optical devices that reflect incident light in a direction which has an essentially fixed angle relative to that of an incident light beam, with little or no dependence on the angle between the light beam and the reflector surface. FIGS. 1a-c, show a directed reflection effect of a directed reflector 20. It is evident that, regardless of the angle of incidence of the incoming-light, the light is reflected at a constant angle with respect to the angle of incidence (xcex94xcex1).
Retroreflectors are directed reflectors which redirect light towards its originating source (i.e., xcex94xcex1=0). A review of various types of retroreflectors and their uses appears in reference [35] which is incorporated herein by reference. This advantageous property of retroreflectors has led to the widespread implementation thereof in planar array configurations which can be utilized in a variety of applications. For example, arrays of miniature retroreflector are often utilized in sheeting which are used for road signs in order to increase their visibility to motorists at night and for retroreflective safety devices used in vehicles and by other road users. Retroreflector arrays are also used with light barriers and with a beam scanning apparatus, such as the beam scanning apparatus used for generating light grids and light curtains.
One type of a retroreflectors array is constructed from a monolayer of microspheres embedded in a cover layer. Behind the rear surfaces of the microspheres, separated by a spacing layer, a reflective layer, e.g., vapor-coated aluminum, is disposed such that light penetrating through, and directed by, the microspheres is reflected out by the reflective layer. Such embedded microspheres array is also referred to in the art as embedded lens array, whereas each microsphere or lens in combination with its reflective surface is sometimes referred to as a cat eye retroreflector element. Nevertheless such microspheres typically have very poor lenticular quality, and as such lack a well defined focal length which is required by various reflector applications. Furthermore, such microspheres are characterized by high level of optical aberrations which result in loss of the ability to focus incident light, particularly when incident at a wide range of acceptance angles, on the reflective layer. Arrays of microsphere based retroreflectors have been observed in nature [16] and have found applications in-man made devices [1]. Such retroreflectors are thoroughly analyzed in the scientific and technical literature [see, for example, references 4, 11, 15, 18, 19, 22 and 51]. There are also several descriptions of fabrication methods for producing sheets containing many miniature cat-eye retroreflectors [9, 10, 17, 40, 42, 49, 50].
As described above, a prior art embedded lens array type retroreflector is typically a combination of crude lenses and a reflector surface (a mirror or a diffuse reflecting surface) that is located at the back focal plane of each lens. Any collimated beam of light that enters this structure is reflected back at the source (retroreflected), provided the reflection from the reflector re-enters the aperture of the same lens. However, embedded lens array retroreflectors utilizing flat, specular (optically smooth) reflectors suffer from a limited range of incidence angles acceptable for retroreflection (also referred to in the art as acceptance angles). This characteristic is demonstrated by FIGS. 2a-b which depict cross sectional views of such retroreflecting arrays. As shown in FIG. 2a, light entering prior art embedded lens array retroreflectors utilizing specular (smooth) reflecting surface, at an angle greater than the acceptance angle, is no longer returned to the same lenslet and as such is not retroreflected. In addition, when the reflecting surface is diffuse (optically rough or otherwise scattering) as specifically shown in FIG. 2b, this diffusely reflective surface of such prior art lens arrays also leads to scattering of the reflected light over several lenses, resulting in loss of retroreflected light and low retroreflection coefficient.
Although this limited angular response range and low retroreflection coefficient of prior art retroreflectors is acceptable for some applications, there are, however, applications for which these effects are non-tolerable. In particular, applications which require accurate reflection of incident light, such as incorporation of retroreflectors in beam scanning applications, are extremely difficult to effect with arrays utilizing embedded microspheres; for these applications it is particularly important that the retroreflector and the light transmitter/receiver of the beam scanning apparatus are accurately aligned.
Since embedded lens arrays retroreflectors are not applicable in various applications such as, for example, beam scanning, interest has been centered, in the past, on cube-corner retroreflectors, which are products of high optical quality and as such can provide more accurate reflected beam of light. Cube-corner retroreflectors are trihedral structures which have three mutually perpendicular lateral faces meeting at a single corner, such as that configuration defined by the corner of a cube. The retroreflectivity typically achieved by cube-corner type reflecting elements is through triple reflection (often utilizing the principle of total internal reflection). A transparent cube-corner element receives a ray of incident light at an angle and sends it back in the same direction. To this end, see, for example, U.S. Pat. Nos. 3,924,929, 4,672,089, 4,349,598, and 4,588,258, EP 0 844 056 A1, which are incorporated herein by reference.
In order to overcome the relatively pronounced directional dependence which is associated with reflection at retroreflectors, attempts have been made to sub-divide a retroreflector consisting of triple reflectors into individual elements (see, for example, DE-PS No. 22 36 482, which is incorporated herein by reference). In this arrangement the individual retroreflecting elements are inclined to one another at increasing angles such that the scanning light beam of a light curtain impinges as closely as possible to normal incidence on the individual triple elements which are directionally dependent. An array configuration with retroreflecting elements inclined at increasing angles to one another is, however, only suitable for use with a sector-shaped scanning beam. Each individual retroreflector element must also be set at a predetermined position relative to the scanning beam and this makes it difficult, if not practically impossible, to manufacture such a retroreflector using cost-effective mass production techniques.
In addition, due to the functional design of cube-corner retroreflectors, fabrication or utilization thereof in sheeting results in the addition of undesirable thickness which limits their applicability.
While retroreflectors can and are manufactured by classical optical procedures (polishing, etc.), such devices are too expensive for most applications. Most retroreflecting surfaces in use today are stamped, molded, or otherwise replicated, arrays of corner-cube elements or cat-eye elements, or a suspension of very small glass or transparent plastic beads (microspheres) in paint, where the beads act as crude lenses and the paint as a reflector. Retroreflectors manufactured by classical methods are high precision instruments of very high performance, but they are too expensive and often too bulky for most applications. The standard molded/stamped retroreflectors of today and, more so, the bead suspension type of retroreflectors are inexpensive, but for many important applications their performance is, at best, marginal.
There is thus a widely recognized need for, and it would be highly advantageous to have, a directed reflector, retroreflector in particular, devoid of the above limitations.
According to one aspect of the present invention there is provided a wide angle directed reflector. The directed reflector according to this aspect of the invention includes a lenticular layer including at least one array of lenslets, each of which having a focal length. The directed reflector according to this aspect of the present invention further includes a reflective layer, which is disposed relative to the lenticular layer. The lenticular layer and the reflective layer are constructed, designed and relatively disposed such that light incident at an angle of incidence on the lenticular layer is reflected by the reflective layer and redirected through the lenticular layer at a substantially constant angle relative to the angle of incidence.
According to another aspect of the present invention there is provided a wide angle controllable directed reflector. The directed reflector according to this aspect of the invention includes a lenticular layer including at least one array of lenslets, each of which having a focal length. The directed reflector according to this aspect of the present invention further includes a reflective layer disposable relative to the lenticular layer. The lenticular layer and the reflective layer are constructed, designed and relatively disposed such that, for a given relative orientation thereof, light incident at an angle of incidence on the lenticular layer is reflected by the reflective layer and redirected is through the lenticular layer at a substantially constant angle relative to the angle of incidence The directed reflector according to this aspect of the present invention further includes a directed reflection controlling mechanism which is designed and constructed to controllably alter the given relative orientation of, or the distance between, the lenticular layer and the reflective layer, such that light incident at the angle of incidence on the lenticular layer is reflected by the reflective layer and redirected through the lenticular layer at an angle different than the substantially constant angle relative to the angle of incidence.
According to further features in preferred embodiments of the invention described below, the directed reflection controlling mechanism is designed and constructed so as to reciprocally alter the given relative orientation of, or the distance between, the lenticular layer and the reflective layer.
According to yet another aspect of the present invention there is provided a light modulating directed reflector. The directed reflector according to this aspect of the present invention includes a lenticular layer including at least one array of lenslets, each of which having a focal length. The directed reflector according to this aspect of the present invention further includes a reflective layer disposed relative to the lenticular layer. The lenticular layer and the reflective layer are constructed, designed and relatively disposed such that light incident at an angle of incidence on the lenticular layer is reflected by the reflective layer and redirected through the lenticular layer at a substantially constant angle relative to the angle of incidence. The directed reflector according to this aspect of the present invention further includes a light modulating layer disposed relative to the lenticular layer and the reflective layer and which serves for modulating light passing through and reflected from the light modulating wide angle directed reflector.
According to still another aspect of the present invention there is provided a light modulating wide angle directed reflector. The directed reflector according to this aspect of the present invention includes a lenticular layer including at least one array of lenslets. The directed reflector according to this aspect of the present invention further includes a reflective layer disposed relative to the lenticular layer. The lenticular layer and the reflective layer are constructed, designed and relatively disposed such that light incident at an angle of incidence on the lenticular layer is reflected by the reflective layer and redirected through the lenticular layer at a substantially constant angle relative to the angle of incidence. The directed reflector according to this aspect of the present invention further includes a light modulating layer disposed relative to the lenticular layer and the reflective layer. The light modulating layer serves for modulating light passing through and reflected from the light modulating wide angle directed reflector. The light modulating according to this aspect of the present invention is transformable from a first light modulating state to a second light modulating state and vice versa.
According to an additional aspect of the present invention there is provided an object identification system. The system according to this aspect of the present invention includes an identifier tag which is mountable on an object to be identified. The identifier tag includes the features described herein for the light modulating directed reflector. The system according to this aspect of the present invention includes an interrogator device. The latter includes (i) a light source for directing light onto the identifier tag; (ii) a light detector for receiving modulated light reflected from the identifier tag; and (iii) a processing unit communicating with the light detector for processing the light reflected so as to yield information corresponding to the light modulating layer, to thereby identify the object.
According to yet an additional aspect of the present invention there is provided a movement monitoring system for monitoring a movement of an object. The system according to this aspect of the present invention includes at least one wide angle directed reflector being mountable on the object, the at least one wide angle directed reflector includes all the features described hereinabove with respect thereto. The system according to this aspect of the present invention further includes an optical sensing device. The latter includes (i) a light source for illuminating the at least one wide angle directed is reflector; and (ii) a detector for receiving light reflected from the at least one directed reflector. The detector serves for monitoring changes in the reflected light, so as to monitor a movement and/or position of the object. The at least one wide angle directed reflector preferably further includes a light modulating layer disposed relative to the lenticular layer and the reflective layer. The light modulating layer being for modulating light passing through and reflected from the light modulating wide angle directed reflector. Preferably, a plurality of wide angle directed reflectors, typically retroreflectors, spaced along the object, such as a wing of an aircraft, are employed, thereby monitoring vibrational movement along the object.
According to further features in preferred embodiments of the invention described below, the reflector is a rear telecentric reflector.
According to still further features in the described preferred embodiments the angle of acceptance of the wide angle retroreflector is at least +/xe2x88x92 20xc2x0 degrees from a normal incidence angle. According to still further features in the described preferred embodiments the substantially constant angle is substantially zero and therefore the directed reflector is a retroreflector.
According to still further features in the described preferred embodiments the substantially constant angle is substantially different than zero and therefore the directed reflector is not a retroreflector.
According to still further features in the described preferred embodiments the reflective layer includes an array of concave reflective elements.
According to still further features in the described preferred embodiments each of the lenslets of the lenticular layer includes a convex face and an opposing flat face.
According to still further features in the described preferred embodiments each of the lenslets of the lenticular layer is selected from the group consisting of a diffractive lenslet, a refractive lenslet, and a combination diffractive-refractive lenslet.
According to still further features in the described preferred embodiments lenslets of the at least one array of lenslets and the concave reflective elements of the array of concave reflective elements are optically co-aligned.
According to still further features in the described preferred embodiments a distance between a lenslet of the lenslets and a respective concave reflective element of the concave reflective elements is selected such that a center of the concave reflective element is located at a focal plane of the lenslet.
According to still further features in the described preferred embodiments the lenticular layer and the reflective layer are integrated into a single layer.
According to still further features in the described preferred embodiments the lenticular layer includes an external array of lenslets and an internal array of lenslets.
According to still further features in the described preferred embodiments each lenslet of each of the external array of lenslets and the internal array of lenslets includes a convex face and a planar opposing face.
According to still further features in the described preferred embodiments respective lenslets of the external array of lenslets and the internal array of lenslets form lenslet pairs such that the planar opposing faces thereof face one another.
According to still further features in the described preferred embodiments the reflective layer is planar.
According to still further features in the described preferred embodiments for each of the lenslet pairs, incident light passing through an aperture of a lenslet of the external array of lenslets and then through a respective internal lenslet of the internal array of lenslets is rendered normal to the reflective layer.
According to still further features in the described preferred embodiments each lenslet of the array of lenslets includes a pair of opposing convex faces.
According to still further features in the described preferred embodiments the reflective layer is a corrugated planar reflector.
According to still further features in the described preferred embodiments the reflective layer includes an array of concave corrugated reflective elements.
According to still further features in the described preferred embodiments each of the lenslets has a diameter less than about 1 mm.
According to still further features in the described preferred embodiments each of the lenslets has a diameter less than about 0.5 mm.
According to still further features in the described preferred embodiments each of the lenslets has a diameter less than about 0.1 mm.
According to still further features in the described preferred embodiments the light modulating layer and the reflective layer are integrated into a single reflective and light modulating layer.
According to still further features in the described preferred embodiments the light modulating layer is selected from the group consisting of a light polarizing layer, a polarization fractional-wave retardation plate, an optical filter, and a patterned layer having substantially opaque regions and substantially transparent regions.
According to still further features in the described preferred embodiments the modulating layer is disposed between said lenticular layer and said reflective layer.
According to still further features in the described preferred embodiments the modulating layer forms a part of said reflective layer.
According to still further features in the described preferred embodiments the light modulating layer is disposed between the lenticular layer and the reflective layer.
According to still further features in the described preferred embodiments the light modulating layer is disposed in front of the lenticular layer.
According to still further features in the described preferred embodiments the light modulating layer includes a fluid.
According to still further features in the described preferred embodiments the patterned layer is a code identifying an object with which the light modulating wide angle directed reflector is associatable.
According to still further features in the described preferred embodiments the code is a bar-code.
According to still further features in the described preferred embodiments the light modulating layer is transformable from a first light modulating state to a second light modulating state and vice versa.
According to still further features in the described preferred embodiments the second light modulating state is neutral.
The present invention successfully addresses the shortcomings of the presently known configurations by providing directed reflectors and retroreflectors which are readily manufacturable and which are characterized by a wide angle of acceptance and high reflection coefficient as is compared to prior art designs.